Thanks Greg.
Some comments inline...
On Sunday, August 18, 2013, Guang Yang wrote:
Hi ceph-users,
>This is Guang and I am pretty new to ceph, glad to meet you guys in the
>community!
>
>
>After walking through some documents of Ceph, I have a couple of questions:
> 1. Is there any comparison between Ceph and AWS S3, in terms of the ability
>to handle different work-loads (from KB to GB), with corresponding performance
>report?
Not really; any comparison would be highly biased depending on your Amazon ping
and your Ceph cluster. We've got some internal benchmarks where Ceph looks
good, but they're not anything we'd feel comfortable publishing.
[Guang] Yeah, I mean the solely server side time regardless of the RTT impact
over the comparison.
2. Looking at some industry solutions for distributed storage, GFS / Haystack
/ HDFS all use meta-server to store the logical-to-physical mapping within
memory and avoid disk I/O lookup for file reading, is the concern valid for
Ceph (in terms of latency to read file)?
These are very different systems. Thanks to CRUSH, RADOS doesn't need to do any
IO to find object locations; CephFS only does IO if the inode you request has
fallen out of the MDS cache (not terribly likely in general). This shouldn't be
an issue...
[Guang] " CephFS only does IO if the inode you request has fallen out of the
MDS cache", my understanding is, if we use CephFS, we will need to interact
with Rados twice, the first time to retrieve meta-data (file attribute, owner,
etc.) and the second time to load data, and both times will need disk I/O in
terms of inode and data. Is my understanding correct? The way some other
storage system tried was to cache the file handle in memory, so that it can
avoid the I/O to read inode in.
3. Some industry research shows that one issue of file system is the
metadata-to-data ratio, in terms of both access and storage, and some technic
uses the mechanism to combine small files to large physical files to reduce the
ratio (Haystack for example), if we want to use ceph to store photos, should
this be a concern as Ceph use one physical file per object?
...although this might be. The issue basically comes down to how many disk
seeks are required to retrieve an item, and one way to reduce that number is to
hack the filesystem by keeping a small number of very large files an
calculating (or caching) where different objects are inside that file. Since
Ceph is designed for MB-sized objects it doesn't go to these lengths to
optimize that path like Haystack might (I'm not familiar with Haystack in
particular).
That said, you need some pretty extreme latency requirements before this
becomes an issue and if you're also looking at HDFS or S3 I can't imagine
you're in that ballpark. You should be fine. :)
[Guang] Yep, that makes a lot sense.
-Greg
--
Software Engineer #42 @ http://inktank.com | http://ceph.com
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